Conventionally, various hard carbon coating films have been known that are formed on sliding surfaces of sliding members, such as piston rings, and have low friction and wear resistance.
The following Patent Literature 1 describes a hard carbon coating film of a piston ring which has a carbon-based coating formed over its sliding surface, wherein the coating is a multilayer coating including two types of layers having different hardnesses laminated in at least two layers, a hardness difference between the two types of layers is 500 to 1700 HV, a high hardness layer has a thickness identical to or greater than that of a low hardness layer, and the coating as a whole has a thickness of 5.0 μm or more.
The following Patent Literature 2 discloses a DLC coating of a sliding element for an internal combustion engine, particularly a piston ring, including a ta-C type DLC coating wherein the residual stress varies over the thickness of the coating to define at least one residual stress gradient.